Zeolite lattice collapse

X-ray diffractometry, spectrometric techniques ( Si high-resolution NMR, adsorbed 125Xe NMR, and electron paramagnetic resonance), and transmission electron microscopy (TEM) have been used to study solid-state reactions occurring between NaY zeolite and V205 at 700 K. When the ratio R (V atom number/[Al + Si] atom number) is 0.2, the zeolite lattice collapses. However, when R 0.05, the lattice remains. The interaction is interpreted in terms of sodium vanadate (containing Vv and atoms) formation. 

NaX (13X) zeolite is the catalyst of choice for benzylic chlorinations while zeolites with high Bronsted acidity (ZF520) affected ring chlorination, even though X-ray diffraction studies have later shown that the zeolite lattice collapses under the reaction conditions127. In both instances the mechanism involves active site outside the channel network of the microporous solid. Contradictory to the latter authors, Delude and Laszlo suggest that aluminum-rich zeolites would preferably initiate radical chain reaction via formation of siloxy radicals. Both the reaction medium and substituents on the aromatic substrate have a profound effect on the rate and selectivity of these reactions. Interestingly, the catalyst applied in the radical chlorinations can be easily recycled and reused. The opposite has been observed in the ionic chlorinations where the catalyst has rapidly lost its activity. 

TJirst studies (1, 2) of ion-exchanged forms of zeolite A reported that the exchange of Na for certain ions caused the breakdown of the zeolite lattice. However, Sherry and Walton (9) reported the existence of a hydrated BaA and concluded that the earlier reports of the nonexistence of this ion-exchanged form were based upon x-ray examination of calcined samples. Dyer, Gettins, and Molyneux (7) confirmed the existence of BaA and were able to measure Ba cation self-diffusion parameters in A. They also concluded that removal of water even at temperatures below 100 °C caused lattice collapse. Recently, Radovanov, Gacinovic, and Gal (8) have reported the preparation of Co(II)A in hydrated form, again contrary to the original studies. 

Fig. 8. a Isothermal DTA trace showing the endotherm connected with the Na-Pl Na-P2 transformation b DTA curve of Ba-exchanged Linde A zeolite, showing the sharp endotherm connected with lattice collapse (reproduced by permission from [35])... 

The concept zeolites conventionally served as the synonym for aluminosilicates with microporous host lattice structures. Upon removal of the guest water, zeolites demonstrate adsorptive property at the molecular level as a result they are also referred to as molecular sieves. Crystalline zeosils, AlPO s, SAPO s, MAPO s (M=metal), expanded clay minerals and Werner compounds are also able to adsorb molecules vitally on reproval of any of the guest species they occlude and play an Important role in fields such as separation and catalysis (ref. 1). Inclusion compounds are another kind of crystalline materials with open framework structures. The guest molecules in an inclusion compound are believed to be indispensable to sustaining the framework structure their removal from the host lattice usually results in collapse of the host into a more compact crystal structure or even into an amorphous structure. 

Lattice imaging studies by TEM on a "young" fraction showed extensive regions of crystallinity with minimal evidence of crystallite fracturing. By contrast, similar investigations of an "old" fraction, combined with in situ compositional analysis, revealed small "islands" of crystallinity within a "sea" of disordered material that was once crystalline. Fracture lines at crystallite boundaries are absent. Instead, the small USY crystallites within the "old" fraction are in intimate mixture with the collapsed zeolite. 

Figure 11. (a) Lattice image showing extent of a FAU crystallite in Fraction B, and (b) Intimate mixture of small FAU crystallites and collapsed zeolite in Fraction F. 

The relative crystalline stability of these materials was evaluated by DTA/TG (Fig.2). It is seen that the exothermic peak (indicating the collapse of the faujasite lattice) around 1133 K in NaY( Sample A) progressively shifts to lower temperataures with increasing iron content in the zeolite reaching 1033 K for Sample E indicating that the presence of iron reduces the thermal stability of the lattice... 

Ti-Beta zeolites and, even more, mesoporous Ti-siUcates can be somewhat unstable to aqueous hydrogen peroxide and to strongly chelating agents. A partial collapse of the lattice and the release of Ti, in the form of Ti02 particles or soluble Ti peroxides, was sometimes observed under these conditions (see also Section 18.4.2). The structural instability grows in parallel with the hydrophiUcity of the surface and the defectiveness of the silica matrix Ti-P < Ti,Al-P Ti-MCM-41 [87-89]. For the same reason, the stability of the catalyst is indirectly related to the method of synthesis, as far as this is able to produce materials with a different content of connectivity defects. 

The collapse to an amorphous phase is the most pronounced effect accompanying treatments of zeolites at elevated temperatures. In addition, heating results in changes in the framework composition, caused by movement of the tetrahedral aluminum into extra-lattice positions. 

Ca in EAH-USY did not evidently affect the crystal lattice constant. Reduction of Ni oxide in EAH-USY zeolite required higher temperature than Fe and V. V interacted with EAH-USY at high temperature and completely destroyed the structure. Fe destroyed the zeolite structure over a wide temperature range, but the structure of EAH-USY did not completely collapse due to the interaction between Fe and the zeolite. 

It is obvious that dealumination of aluminum-rich zeolite frameworks resulting in the formation of high lattice defect concentrations should diminish the stability of the crystal structure. As early as 1958 it was reported [21] that the structure of faujasite-type zeoHtes collapsed completely upon treatment with strong mineral acids. However, Lee and Rees [22] have shown that the crystal structure of Y zeolite is not significantly affected if the amount of HCl appHed in aqueous solutions does not exceed 10 mmol/g Na-Y which results in the release of 56% of the framework aluminum atoms and in the complete exchange of the sodium cations. Thus, at least part of the aluminum in Na-Y zeoHte can be extracted without considerable lattice destruction if HCl is applied in amounts that do not yet cause too intense dealumination. 

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